Due to the constant demand for improved efficiency of antennas and increased battery lifetime in portable communication systems high-impedance surfaces have been the subject of increasing research. High-impedance surfaces have a number of properties that make them important for applications in communication equipment. The high-impedance surface is a lossless, reactive surface, whose equivalent surface impedance,       Z    s    =            E              t        ⁢                  xe2x80x83                ⁢        an                    H              t        ⁢                  xe2x80x83                ⁢        an            
(where Etan is the tangential electric field and Htan is tangential magnetic field), approximates an open circuit. The surface impedance inhibits the flow of equivalent tangential electric surface current and thereby approximates a zero tangential magnetic field, Htan≈0.
One of the main reasons that high-impedance surfaces are useful is because they offer boundary conditions that permit wire antennas (electric currents) to be well matched and to radiate efficiently when the wires are placed in very close proximity to this surface. Typically, antennas are disposed less than xcex/100 from the high-impedance surfaces (usually more like xcex/200), where xcex is the wavelength of operation. The radiation pattern from the antenna on a high-impedance surface is substantially confined to the upper half space, and the performance is unaffected even if the high-impedance surface is placed on top of another metal surface. The promise of an electrically-thin, efficient antenna is very appealing for countless wireless device and skin-embedded antenna applications.
One embodiment of a conventional frequency selective surface (FSS) is shown in FIG. 1. The FSS acts like thin high-impedance surface within a particular frequency range, or set of frequency ranges. It is a printed circuit structure, using an electrically-thin, planar, periodic structure, with vertical and horizontal conductors, which can be fabricated using low cost printed circuit technologies. The combination of the FSS with a ground backplane is known as an artificial magnetic conductor (AMC). Near its resonant frequency the AMC approximates an open circuit to a normally incident plane wave and suppresses TE and TM surface waves over the band of frequencies near where it operates as a high-impedance surface.
An antenna, such as bent-wire monopole, may be disposed within close proximity to the surface of the AMC, thus decreasing the overall thickness of the device. Bent-wire monopoles are primarily used as the antenna element that is integrated with an AMC. The bent-wire monopole is simply a thin wire or printed strip located a small fraction of a wavelength about xcex/200 above the AMC surface. The bent-wire monopole is disposed on the AMC surface using a thin layer of low loss dielectric material. Typically, a coaxial connector feeds one end of this strip antenna. The outer conductor of the coaxial connector is soldered to the conducting backplane of the AMC, and the inner conductor extends vertically through the AMC and a thin dielectric layer upon which the monopole is printed or disposed to connect to the monopole. Measurements of one such unloaded antenna including the E-plane and H-plane gain patterns at several L-band frequencies are shown in FIGS. 2(a) and 2(b), respectively. This AMC antenna included an unloaded 1.64 inch (4.17 cm) long by 0.050 inch (0.127 cm) wide bent-wire monopole mounted on 1.5 inch (3.81 cm) by 2.5 inch (6.35 cm) AMC with a resonant frequency near 1.8 GHz.
However, one drawback of such an antenna is that the monopole must have an electrical length of one-quarter of a wavelength, which makes integration of the AMC antenna into a handheld device more of a challenge as devices decrease in size. To reduce the length of the antenna for a given frequency of operation, an inductor can be placed in series with the monopole near the feed point of the antenna, i.e. where the coaxial connector attaches to the monopole, to reduce the length of the antenna for a given frequency of operation. Either printed inductors, which are integrated with the printed monopole, or chip inductors may be used.
However, inductors have a number of problems. One of these problems includes a large amount of loss in the antenna, which results in a relatively inefficient antenna. The reduction in antenna gain increases the power consumption and decreases the battery life of the device. In addition, chip inductors are relatively expensive and bulky in comparison with the monopole. Examples of the E-plane and H-plane gain patterns at several L-band frequencies of typical chip inductance-loaded antennas are illustrated in FIGS. 3(a) and 3(b), respectively. This AMC antenna included a 1 inch (2.54 cm) long bent-wire monopole base-loaded with a 7 nH inductor mounted on 1.5 inch (3.81 cm) by 2.5 inch (6.35 cm) AMC with a resonant frequency near 1.8 GHz. Although the length of the antenna has been reduced to 60% of its original size by inductive loading, the gain has been reduced between a minimum of about 1.5 dB to a maximum of about 8 dB, depending on the frequency and principal plane, as compared with an unloaded antenna. In general, however, the loss when inserting the inductor may be limited to 1-3 dB. These correspond to efficiencies of from 70% to 16% compared to that of an unloaded antenna. One factor that results in the reduction in efficiency is the windings of the chip inductor, which contribute dissipative loss. Another factor that degrades the antenna efficiency is the mismatch between the impedance of the antenna and that of the inductor. The fabrication of a reduced-size, non-inductively loaded antenna having high efficiency would be of great value.
To reduce the length of an antenna element, such as a bent-wire monopole, relative to an unloaded antenna, increase the radiation efficiency and battery life in portable devices, and fabricate low cost antennas, one embodiment of the antenna comprises an artificial magnetic conductor (AMC), an antenna element disposed on the AMC and having a feed, and a capacitive load separated from the feed and connected with the antenna element.
The capacitive load may be disposed at an end of the antenna element and may be any of: a lumped capacitive load, a distributed capacitive load, a surface mounted capacitive load or a capacitive patch (part of a printed trace or separate metal). The capacitance may have a fixed value or be variable. The reduction in gain between an antenna element without the capacitive load and with the capacitive load may be at most 5 dB. If the capacitive load is lumped, the lumped capacitive load may be connected with an RF backplane of the AMC through a dedicated connection to the backplane or to at least one grounded conductive portion of the FSS that is contained within the AMC.
The capacitive load may form a capacitance between the antenna element and the backplane or between the antenna element and a grounded conductive portion of the FSS. The capacitive load may be in excess of that of the per unit length capacitance of the antenna element.